NITZSCHIA SINGALENSIS VS N. FIRTHII

What’s in a diatom’s name?

Frithjof A.S. Sterrenburg

Email: fass AT wxs DOT nl

with contributions by David Walker

In the January 2010 issue of Micscape, Dave Walker described his investigations of a very finely structured Nitzschia. Fearing to become trapped in the minefield of diatom taxonomy, he apologized in advance for “any errors e.g. of interpretation, misidentification, nomenclature or insights”. His fears were not really necessary, there are some boobytraps attached to the question of Nitzschia singalensis vs N. firthii, but they were not of his doing. Naming species is sometimes regarded as an abstruse archivarian’s activity, but this view is nonsense. No sensible scientific discussion or investigation can take place if the identity of the subject of study is not unequivocally fixed. Similarly, addressing your spouse by another name would cause serious complications, so here are some remarks on Walker’s article.

Nomenclature is the naming of an organism. The question whether it should be “singalensis” or “singalense” is a nomenclatural one (the former is correct, as Nitzschia is feminine, e.g. N. sigmoidea).

Taxonomy relates to the position of the organism within the realm of living things, its (biological) identity. To guarantee reliable future reference, this identity should be securely and unambiguously linked to a name. For diatoms, which are often difficult to identify and whose legions include more species than all other groups of Algae combined, this is particularly critical and a standard procedure must be followed. The taxonomic question here is whether N. singalensis is the same organism as N. firthii.

How to define a diatom species

When a new diatom species is described, the author must publish a description (diagnosis) in a peer-reviewed journal that contains the data by which the species can be recognized and separated from all other species already described. This description must be in Latin; a translation into some other language is always given but is not enough by itself. Illustrations must be supplied showing the characters given in the description. The species should be nomenclaturally sound, e.g. correct Latin or Greek derivation, no duplication with an already described species and you can’t name a species after yourself. But most of all: a specimen or specimens from the original material studied (for diatoms this means: one or several slides) must be deposited in a recognized collection. This slide must be designated by the author as the type.

These rules were not yet so strictly enforced in Victorian and Edwardian times. For instance, many species have been introduced in the famous Schmidt Atlas merely in the form of a picture with minimal description. Taxonomists since then have supplied additional data for many of these species and designated types, respecting the original names whenever possible.

The taxonomic issue

Let’s summarize the taxonomic situation (for the literature cited, refer to Walker’s article):

1) Spitta 1920 referred to an organism called N. singalensis and showed photomicrographs illustrating the striae but gave no description.

2) Firth marketed slides labelled N. singalensis but did not publish a description or photomicrograph. The locality of collection is reported as “Amherst, Burma”.

3) Fuge found a Nitzschia in a sample from “Chinese canned fish”, decided that this was the same as Firth’s and Spitta’s N. singalensis but introduced a new name, N. firthii.

4) Needham stated that Fuge’s “Chinese canned fish” sample came from the “stomachs of Chinese canned fish”.

5) McLaughlin concluded that the species in Fuge’s “Chinese canned fish” material was identical to N. geitleri Hustedt.

As regards 1) and 2), these data indeed do not amount to the valid introduction of a new species, one would call it a “manuscript species”. However, the fact that Firth marketed slides labeled “Nitzschia singalensis” and the reference to “Amherst, Burma” as the locality should, in my opinion, serve as the starting-point for any further work. What we do have there is verifiable specimens linked to a specific locality.

That’s why I am not happy with Fuge’s procedure 3). Faced with two diatoms with few visible features permitting separation, he concluded that they were identical and then introduced a new name. However, he studied a totally different sample, “Chinese canned fish”, and that’s a different kettle of fish altogether if you’ll excuse my pun. So doing, he introduced a paradigm shift, whilst continuity could have been assured by referring to a tangible entity: a Firth slide. He must have had such a slide available as he concluded that the Burma specimens were identical to the canned fish specimens – it would have been irresponsible to draw this conclusion merely based on the illustrations available.

So it would have been elegant and far preferable if Fuge had validated Firth’s manuscript species. He could simply have designated the Firth slide he had examined as the type of the valid species Nitzschia singalensis Firth emend. Fuge, the “emend.” meaning : “as corrected by”. That would have resulted in continuity of identification and Walker was quite right when he decided to continue to use the name N. singalensis in quotes (rather than N. firthii) when referring to his own studies of slides marked with that name.

This Chinese canned fish sample greatly puzzles me anyway. It’s not clear how the diatoms came to be canned too. Needham’s remark 4) is informative in so far as fish stomachs can indeed yield large crops of diatoms, but I cannot believe that the Chinese would have canned fish without gutting them first!

The dilemma of whether the specimens in the canned fish sample are identical to those in the Burma sample is unresolved. This would absolutely require examination in SEM, which has revealed major structural differences in scores of diatoms that look alike in the LM. This also applies to McLaughlin’s view 5) that the organism we’re discussing is the same as N. geitleri. They do look similar, but mainly because there’s so little to see! In addition, there are major differences in their geography and biotope: N. singalensis is from the tropics and marine, while N. geitleri occurs in waters such as the Neusiedler See in Austria, which is temperate (and very cold in winter) and only mildly saline.

Trick or treat?

Some remarks on the structures shown in Walker’s images:

It’s evident that the exotic and poisonous mountant Realgar yields superb images. I have compared a few Realgar slides from museum collections with slides of the same material in Naphrax and despite modern optical techniques such as DIC, images from the latter were always inferior.

Secondly, the puncta (“dots”) Walker managed to resolve are real, not artefacts. From the LM images, the diatom is clearly a Nitzschia and all Nitzschias that have been examined in SEM show areolae (perforations), not costae (solid bars). Optical artefacts originating in imperfect resolution simulate continuous lines when there are actually rows of dots, not conversely.

For the structures at the apex of the valve, e.g. the “fork” referred to by McLaughlin 5) and arrowed in one of Walker’s images, the situation is different. This is evidently a three-dimensional structure, which is impossible to interpret in the light-microscope and requires SEM.

Finally: the puncta shown in the illustration in Fuge’s paper. It should be noted that this drawing was not made by Fuge, but by the late Norman Hendey (as stated in Fuge’s paper). Hendey was a very critical and extremely competent diatomist and as he drew puncta, he must have observed them somehow, if only faintly. Perhaps he used a particularly coarsely structured specimen for the illustration?

Comments to the author, Frithjof Sterrenburg, are welcomed.

*****

Micscape Editor David Walker's acknowledgement. I would like to thank Frithjof for explaining puzzling aspects of diatom naming and interpreting fine structure of the Nitzschia species as raised in my January 2010 article.

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